The cleaner head of a vacuum cleaner typically comprises an agitator driven by a drive assembly. The drive assembly may include a turbine for generating the torque necessary to drive the agitator. The turbine may be driven by dirty air that is drawn in through the main suction inlet. Alternatively, the turbine may be driven by clean air that is drawn in through an inlet distinct from the suction inlet.
A clean-air turbine has the advantage that the turbine is not exposed to the dirty air, which might otherwise obstruct the turbine. However, should the path carrying the dirty air become obstructed, the flow of clean air through the turbine will increase. As a result, the speed of the turbine will increase. The increase in speed may be several orders of magnitude greater than the normal operating speed of the turbine. As a result, components of the drive assembly, such as bearings, may be damaged.
Schemes for preventing overspeed of a clean-air turbine are known. However, such schemes tend to be complicated, thereby increasing the cost, size and/or weight of the cleaner head.